1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and its fabrication method, and more particularly, to an LCD device and its fabrication method that prevents degradation of picture quality by forming a storage capacitor, simplifies the fabrication process, and enhances manufacturing yield by reducing the number of masks used to fabricate a thin film transistor (TFT).
2. Description of the Related Art
Recently, as the demand for information displays has increased, especially for use in portable (mobile) information devices, research and development of light thin flat panel displays (FPD), has increased.
Among FPDs, LCDs, exhibit excellent resolution and color and picture quality, so LCDs are widely used in notebook computers, desktop monitors or the like.
The liquid crystal display panel includes a first substrate, namely, a color filter substrate, a second substrate, namely, an array substrate, and a liquid crystal layer formed between the color filter substrate and the array substrate.
In the liquid crystal display, a thin film transistor (TFT) is commonly used as a switching device. An amorphous silicon thin film or a polycrystalline silicon thin film may be used as a channel layer of the TFT.
In fabricating the LCD device, a plurality of masking processes or photolithography process are required to fabricate the LCD device including the TFT, so a method for reducing the number of masking processes is beneficial in increasing productivity.
The structure of a related art LCD device will now be described with reference to FIG. 1.
FIG. 1 is a plan view showing a portion of an array substrate of the related art LCD device. Although actual LCD devices include M×N pixels as the N gate lines and the M data lines cross each other, only one pixel is shown in FIG. 1 for the sake of explanation.
As shown, a gate line 16 and a data line 17 are arranged vertically and horizontally on an array substrate 10, defining a pixel region. A TFT that is a switching device is at the crossing of the gate line 16 and the data line 17. A pixel electrode 18 is formed at each pixel region.
The TFT includes a gate electrode 21 connected to the gate line 16, a source electrode 22 connected to the data line 17, and a drain electrode 23 connected to the pixel electrode 18. The TFT also includes a first insulation film (not shown) and a second insulation film (not shown) for insulating the gate electrode 21 from the source/drain electrodes 22 and 23, and an active area 24 that forms a conductive channel between the source and drain electrodes 22 and 23 when a gate voltage is supplied to the gate electrode 21.
Through the first contact hole 40A formed in the first and second insulation films, the source electrode 22 is electrically connected with a source region of the active area 24 and the drain electrode 23 is electrically connected with a drain region of the active area 24.
A third insulation film (not shown) having a second contact hole 40B is formed on the drain electrode 23, so that the drain electrode 23 and the pixel electrode 18 are electrically connected through the second contact hole 40B.
The process of fabricating the LCD device constructed as described will now be explained with reference to FIGS. 2A to 2F.
FIGS. 2A to 2F are sequential sectional views of the process for fabricating the LCD device of FIG. 1 taken along line I-I′. The illustrated TFT is a polycrystalline silicon TFT that uses polycrystalline silicon as a channel layer.
As shown in FIG. 2A, the active area 24 is formed as a polycrystalline silicon thin film on the substrate 10 by using a photolithography process (a first masking process).
Next, as shown in FIG. 2B, a first insulation film 15A and a conductive metal material are sequentially deposited on the entire surface of the substrate 10 with the active area 24 formed thereon, and then, the conductive metal material is selectively patterned by using the photolithography process (a second masking process) to form the gate electrode 21 over the active area 24 with the first insulation film 15A interposed therebetween.
Thereafter, p+ type or n+ type source/drain regions 24A and 24B are formed at certain regions of the active area 24 by injecting a high density impurity ion or dopant using the gate electrode 21 as a mask. The source/drain regions 24A and 24B are ohmic contact regions that contact the source/drain electrodes.
Then, as shown in FIG. 2C, the second insulation film 15B is deposed on the entire surface of the substrate 10 with the gate electrode 21 and then, a portion of the first and second insulation films 15A and 15B is removed through photolithography (a third masking process) to form the first contact hole 40A exposing a portion of the source/drain regions 24A and 24B.
Subsequently, as shown in FIG. 2D, a conductive metal material is deposited on the entire surface of the substrate 10 and then patterned using photolithography (a fourth making process) to form the source electrode 22 connected with the source region 24A and the drain electrode 23 connected with the drain region 24B through the first contact hole 40A. In this case, a portion of the conductive metal layer constituting the source electrode 22 extends in one direction to form the data line 17.
And then, as shown in FIG. 2E, a third insulation film 15C is deposited on the entire surface of the substrate 10, and then, the second contact hole 40B is formed, exposing a portion of the drain electrode 23 using photolithography (a fifth masking process).
Finally, as shown in FIG. 2F, a transparent conductive metal material is deposited on the entire surface of the substrate 10 with the third insulation film 15C formed thereon and then patterned by using photolithography (a sixth masking process) to form the pixel electrode 18 connected with the drain electrode 23 through the second contact hole 40B.
As described above, in fabricating the LCD device including the polycrystalline silicon TFT, a total of six photolithographs processes are required to form the active area, the gate electrode, the first contact hole, the source/drain electrodes, the second contact hole, and the pixel electrode.
The photolithography process is a process of transferring a pattern formed on a mask onto a thin film deposited on a substrate to form a desired pattern that includes a plurality of processes such as applying a photosensitive solution and exposing and developing processes. As a result, the plurality of photolithography processes degrades the production yield and increases the possibility that a fabricated TFT is defective.
In particular, the masks designed to form the pattern are expensive, so the increasing in the number of masks used in the process leads to an increase in a fabrication cost.